Evidence of vivianite in FeSO4-flocculated sludges

Precipitation of sludges with ferrous iron salts can increase the total phosphorus content of sludges. However, the forms of phosphorus thus precipitated have never been directly investigated. Three dewatered urban sewage sludges were sampled from wastewater treatment plants performing dephosphatation with the addition of FeSO4 (two anaerobically digested sludges and an activated sludge). Phosphorus species in sludges were investigated by X-ray diffraction, 57Fe Mössbauer spectroscopy and scanning electron microscopy. Vivianite, a ferrous phosphate, was observed in the three sludges. In the two anaerobically digested sludges, 60–67% of the total sludge iron was accounted for by an oxidized vivianite. In the activated sludge, 43% of the total iron was accounted for by a non-oxidized vivianite. In addition, calcium phosphate particles were observed in this sludge. The effects of sludge treatment (FeSO4 addition, digestion, dewatering) and air-drying in the laboratory on the formation and oxidation of vivianite are discussed.